Osteoporosis has been recognized as a serious complication of Crohn's disease in adults for several decades. Its importance as a complication of this disease in the pediatric population has been virtually unstudied. The purpose of this study is to examine the prevalence of decreased bone density in children and young adults with Crohn's disease. One hundred sixty-five subjects with the diagnosis of Crohn's disease followed at The Children's Hospital of Philadelphia (CHOP) for their care will be recruited for the study. Each subject will have his or her bone density measured using DEXA (lumbar spine, whole body and hip). Subjects will also undergo anthropometric measurements, nutritional evaluation by a 3-day diet record and calcium food frequency questionnaire, physical activity assessment by use of a questionnaire and laboratory evaluation. This data will allow us to explore the contribution o decreased bone density of disease duration, location and severity, exposure to corticosteroids, dietary intake, nutritional and growth status, level of physical activity and family history of osteoporosis. Subjects with low DEXA measurements will be randomized into a placebo controlled clinical trial of supplemental calcium and followed for six months. Outcomes will include DEXA, anthropometric and dietary measurements.